The solar cells of photovoltaic modules are typically fabricated as separate physical entities with light gathering surface areas on the order of 4-6 cm2 or larger. For this reason, it is standard practice for power generating applications to mount photovoltaic modules containing one or more solar cells in a flat array on a supporting substrate or panel so that their light gathering surfaces provide an approximation of a single large light gathering surface. Also, since each solar cell itself generates only a small amount of power, the required voltage and/or current is realized by interconnecting the solar cells of the module in a series and/or parallel matrix.
A conventional prior art photovoltaic module 10 is shown in FIG. 6. A photovoltaic module 10 can typically have one or more photovoltaic cells (solar cells) 12a-12b disposed within it. Because of the large range in the thickness of the different layers in a solar cell, the cells 12a, 12b, and other cells described herein are depicted schematically. Moreover, FIG. 6 is highly schematic so that it represents the features of both “thick-film” solar cells and “thin-film” solar cells. Typically, solar cells that use an indirect band gap material to absorb light are typically configured as “thick-film” solar cells because a relatively thick film of the absorber layer is required to absorb a sufficient amount of light. Solar cells that use a direct band gap material to absorb light are typically configured as “thin-film” solar cells because only a thin layer of the direct band-gap material is needed to absorb a sufficient amount of light.
The arrows at the top of FIG. 6 show the source of direct solar illumination on the photovoltaic module 10. The layer 102 of the solar cells 12a, 12b is the substrate. Glass or metal is a common substrate. In some instances, there is an encapsulation layer (not shown) coating the substrate 102. In some embodiments, each solar cell 12a, 12b in the photovoltaic module 10 has its own discrete substrate 102 as illustrated in FIG. 6. In other embodiments, there is a substrate 102 that is common to all or many of the solar cells 12a, 12b of the photovoltaic module 10.
The layer 104 is the back electrical contact for each of the solar cells 12a, 12b in the photovoltaic module 10. The layer 106 is the semiconductor absorber layer of each of the solar cells 12a, 12b in the photovoltaic module 10. In a given solar cell 12a, 12b, the back electrical contact 104 makes ohmic contact with the absorber layer 106. In many but not all cases, the absorber layer 106 is a p-type semiconductor. The absorber layer 106 is thick enough to absorb light. The layer 108 is the semiconductor junction partner that, together with the semiconductor absorber layer 106, completes the formation of a p-n junction of each solar cell 12a, 12b. A p-n junction is a common type of junction found in the solar cells 12a, 12b. In p-n junction based solar cells 12a, 12b, when the semiconductor absorber layer 106 is a p-type doped material, the junction partner 108 is an n-type doped material. Conversely, when the semiconductor absorber layer 106 is an n-type doped material, the junction partner 108 is a p-type doped material. Generally, the junction partner 108 is much thinner than the absorber layer 106. The junction partner 108 is highly transparent to solar radiation. The junction partner 108 is also known as the junction partner layer, since it lets the light pass down to the absorber layer 106.
In typical thick-film solar cells 12a, 12b, the absorber layer 106 and the junction partner layer 108 can be made from the same semiconductor material but have different carrier types (dopants) and/or carrier concentrations in order to give the two layers their distinct p-type and n-type properties. In thin-film solar cells 12a, 12b in which copper-indium-gallium-diselenide (CIGS) is the absorber layer 106, the use of CdS to form the junction partner 108 has resulted in high efficiency photovoltaic devices. The layer 110 is a counter electrode that is used to draw current away from the junction since the junction partner 108 is generally too resistive to serve this function. As such, the counter electrode 110 is typically highly conductive and substantially transparent to light. The counter electrode 110 can be a comb-like structure of metal printed onto the junction partner layer 108 rather than forming a discrete layer. The counter electrode 110 is typically a transparent conductive oxide (TCO) such as doped zinc oxide. However, even when a TCO layer is present, a bus bar network 114 is typically needed in the conventional photovoltaic module 10 to draw off current since the TCO has too much resistance to efficiently perform this function in larger photovoltaic modules. The network 114 shortens the distance charge carriers must move in the TCO layer in order to reach the metal contact, thereby reducing resistive losses. The metal bus bars, also termed grid lines, can be made of any reasonably conductive metal such as, for example, silver, steel or aluminum. The metal bars are preferably configured in a comb-like arrangement to permit light rays through the TCO layer 110. The bus bar network layer 114 and the TCO layer 110, combined, act as a single metallurgical unit, functionally interfacing with a first ohmic contact to form a current collection circuit.
An optional antireflective coating 112 allows a significant amount of extra light into the solar cells 12a, 12b. Depending on the intended use of the photovoltaic module 10, it might be deposited directly on the top conductor 110 as illustrated in FIG. 6. Alternatively or additionally, the antireflective coating 112 can be deposited on a separate cover glass that overlays the top electrode 110. In some embodiments, the antireflective coating 112 reduces the reflection of the solar cells 12a, 12b to very near zero over the spectral region in which photoelectric absorption occurs, and at the same time increases the reflection in other spectral regions to reduce heating. U.S. Pat. No. 6,107,564 to Aguilera et al., hereby incorporated by reference herein in its entirety, describes representative antireflective coatings that are known in the art.
The solar cells 12a, 12b typically produce only a small voltage. For example, silicon based solar cells produce a voltage of about 0.6 volts (V). Thus, solar cells 12a, 12b are interconnected in series or parallel in order to achieve greater voltages. When connected in series, voltages of individual solar cells add together while current remains the same. Thus, solar cells arranged in series reduce the amount of current flow through such cells, compared to analogous solar cells arranged in parallel, thereby improving efficiency. As illustrated in FIG. 6, the arrangement of the solar cells 12a, 12b in series is accomplished using interconnects 116. In general, an interconnect 116 places the first electrode of one solar cell 12a in electrical communication with the counter-electrode of an adjoining solar cell 12b of a photovoltaic module 10.
Various fabrication techniques (e.g., mechanical and laser scribing) can be used to segment a photovoltaic module 10 into individual solar cells (e.g., 12a, 12b) to generate high output voltage through integration of such segmented solar cells. Grooves that separate individual solar cells typically have low series resistance and high shunt resistance to facilitate integration. Such grooves are typically made as small as possible in order to reduce dead area and enhance material usage.
Discussion or citation of a reference herein will not be construed as an admission that such reference is prior art to the present application.